1. Field of the Invention
The present invention relates to a light wavelength tuning device for tuning the wavelength of light emitted from an optical device, and a light source, optical demultiplexer and wavelength division multiplexed optical communication system using the tuning device.
2. Description of the Related Art
Recently, there has been a demand for the development of a system for high-density wavelength optical communication to meet the requirement for increase in communication capacity in the optical communication technique. In a conventional wavelength division multiplexed (WDM) optical communication system, for example, light signals from a plurality of light sources are multiplexed by means of an optical coupler or a WDM coupler and transmitted by means of a single-core optical fiber. In this optical communication system, the light signals are transmitted separately for individual wavelengths on the transmission side, and light signals with specific wavelengths, branched by means of an optical demultiplexer, are received on the reception side.
According to this system, however, if the lasing wavelength of light emitted from any of the light sources shifts, for example, the light may possibly interfere with light from any other light source. Therefore, the respective lasing wavelengths of the light sources must be accurately adjusted to a constant value.
As a measure to attain this, there is a light wavelength tuning device that is described in Jpn. Pat. Appln. Publication No. 2546151.
Conventionally, however, an optical filter used in the light wavelength tuning device of this type is formed of a dielectric multilayered film, of which the wavelength characteristic depends much on temperature, so that wavelength tuning cannot be accurate enough.
In a conventional optical demultiplexer, however, the light wavelength tuning device serves only to keep the device temperature constant, and never to adjust the passing wavelength to a fixed value. If the wavelength of light transmitted through the optical demultiplexer shifts in the aforesaid WDM optical communication system of the high-density wavelength type, however, light with a desired wavelength cannot be transmitted, or light with any other wavelength may be transmitted. Possibly, therefore, degradation of transmission quality or transmission failure may occur.
Thus, if the control wavelength for the light wavelength tuning device for each optical device is subject to a substantial temperature error in the WDM optical communication system, degradation in transmission quality, such as a transmission error, is caused in the optical communication system.
A novel lasing wavelength fixing device is described in "Laser wavelength stabilization using holographic filters" (IEEE/LEOS meeting on optical networks and their enabling technologies, Jul. 11-13, 1994, Lake Tahoe, Nev.) by C. Malo, et al. In this device, the lasing wavelength of a laser is fixed by means of a crystal of lithium niobate (LiNbO.sub.3) on which a grating is formed.
It is known, however, that the Bragg wavelength of the grating in this device has temperature dependence of 0.01 nm/.degree. C. The crystal of lithium niobate is placed on a Peltier cooler, and is fitted with a thermistor for detecting temperature. Further, the device is expected to contain therein a lens for collimating light emitted from an optical fiber, beam splitter, condenser, and two photodiodes, thus entailing a complicated optical system. If this device is used, therefore, the device configuration is complicated and large-sized, so that the manufacturing cost is too high.